1. Field of the Invention
This invention relates to loud speakers and in particular to the construction of audio speakers that have virtually no wobble of the voice coil bobbin during operation.
2. Description of the Related Art
A goal of sound reproduction equipment is to provide a life-like sound quality to the listener. Life-like sound quality is understood to be best achieved when a sound system including the speakers have a flat frequency response curve throughout the range of sound frequencies audible to the human ear, generally 20 to 20,000 Hz. A normal speaker cabinet has an electro magnetically driven speaker cone sealed to an opening in the wall of a sealed cabinet. This arrangement provides a drooping frequency response curve (e.g., 22 in the graph 20 of FIG. 1).
The graph 20 of FIG. 1 represents a comparison of sound level verses frequency (i.e., frequency response). The plot 22 shows the drooping response for a closed cabinet system. Over the years, in an effort to improve sound quality low, mid, and high range speakers have been placed in separate cabinets or compartments. Each of those separate cabinets or compartments could then be tuned by creating ports, with or without tubes, in the cabinet to improve the frequency response. At low frequencies, the use of open ports, or open ports and tubes, in the speaker cabinet becomes unmanageable because of the large air mass that needs to be moved to provide adequate tuning. As an example, an ideal cabinet size to hear low frequencies might be larger than the room in which the listener was sitting.
In an effort to offset the effects of a rigid sealed cabinet and avoid the spatial requirements necessary when attempting to create ports or tube ports with speakers low frequencies, passive radiators (generally configured like speakers, but without the electro mechanical driver) have been placed in a secondary opening of the walls of the speaker cavity to reduce the drop-off of the loudness at low frequencies. An example of the improvement in the frequency response when such a passive radiator is installed is shown as plot 24 in FIG. 1. An example of the improvement in the frequency response attributable to the installation of a prior art passive radiator can be understood by reviewing plot 26 in FIG. 2. Note that the drop in the frequency response curve at lower frequencies in plot 26 is very severe before the range of inaudible frequencies 28 is reached. In this configuration, AREA2, the area under the curve to the right of the peak above a minimum loudness level, is larger than AREA1 which is the area under the curve to the left of the peak. This imbalance is indicative of the relative distortion that can be heard as the loudness of the passive radiator nosedives and falls below an audible loudness. The low frequency loudness and energy are not balanced with the high frequency loudness and energy. The area under the curves provide a measure of the imbalance.
Recent trends in the audio systems market have been leaning towards enhancing the bass or sub-woofer response of the audio reproduction systems, so that even if a sound is below the low limit of the range of audible sound, the sound level is high enough so that the listener, although he or she cannot “hear” the sound with ears, they can “feel” the sound as parts of their body are hit by the low frequency waves. At low frequencies, a limitation of passive radiators has been that the low frequencies require large displacements of the moveable radiator elements. Such large displacements can exceed the available range of motion of moveable radiator elements. For example, in FIGS. 4, 5 and 6, a speaker spider 62 at its perimeter is attached to the back end of a speaker basket 50 while the spider's center edge (or core) it is attached to the back end of a speaker cone 58 or a diaphragm 68 to spider 72 connection element 74. In each pictured radiator, a central moveable element is suspended by a speaker “surround” (52, 70, 84) which acts as the flexible element between the stationary front of the speaker basket (50, 66, 80) and the speaker moveable element. Because the range of travel available from each spider (62, 72, 88) is less than the range of travel available from the surround (52, 70, 84), as the spider (62, 72, 88) reaches the limit of its travel and stops. The sudden stop in the movement of the spider, due to its full extensions, causes distortions in adjacent components as well as in the pressure gradients in the speaker chamber. These distortions can be heard as static and/or unnatural discontinuities in the sound. The ratio of the speaker basket back opening “B” (which supports the spider) to the speaker basket front opening “A” (which supports the surround) is approximately 0.5 (or 50%).
In the instance when a passive radiator constructed solely of a speaker cone is connected only as its peripheral rim to an annular support surface in the wall of a speaker, for example, as shown in the Klasco U.S. Pat. No. 4,207,963, a larger range of travel is available to accommodate large movable element displacements experienced at high volume and low frequencies. However, the use of a surround around the perimeter of the top of the cone and the cone shape produces cone wobble which also distorts the sound. The object of the Klasco patent was to arrange active elements to reduce the wobble in the passive radiator.
In the instance where a lone speaker cone suspended in a cavity opening is used, the response of the passive radiator during low frequency cycles as the cone is forced outward and pulled inward can be non-linear as the flexible member (surround) holding the cone tends to have different non-linear force to displacement  characteristics when being stretched outwardly as compared to when it is being stretched inwardly.
The limitations on travel as shown in the prior art described in FIGS. 4, 5 and 6 and the wobble of a passive radiator as discussed in the Klasco patent and such a configuration's non-linearity, highlight the shortcomings of the prior art passive radiators.
The spatial requirement of the prior art passive radiators is also a drawback. The prior art passive radiators are quite large and bulky and extend a large distance into any sealed cavity. This spatial requirement must be taken into account when designing features and companion speakers to fit into the sealed cavity.
Recently there has been an increasing demand for loudspeakers for use in a very compact/shallow space. This demand was born by consumer appetite for louder sound grew couple with the desire for less obtrusive speakers. Recently, home audio consumers have begun a major shift from larger, conventional loudspeakers housed in cabinets that stand alone in the room—to smaller piston speakers that mount within the wall of a house. The available depth in in-wall locations is dictated by the use of 2×4 studs during construction thus creating a space that is less than 4″ deep.
This need for shallow, low profile speakers are not limited to meeting the home audio demand. Such low profile speakers also have application in cars, boats, airplanes and other locations that will benefit from the depth reduction without taxing the sound pressure level. In cars for example, the available mounting depth behind the door panel is much less than the minimum height of conventional speakers. In order to use conventional speakers in such locations, it is nearly always necessary to use a raised grill cover over the speaker since it necessary to have a portion of the speaker height extend above the surface of the door panel into the passenger compartment.
For the most part, subwoofer construction has followed conventional technology—the use of an oscillating diaphragm that responds to a varying magnetic field developed by an applied audio signal. That varying magnetic field causes the diaphragm to be attracted and repelled to and from the intermediate position where the diaphragm rests when no audio signal is applied to the speaker. For the most part, current speaker technology uses a loudspeaker made of a rigid diaphragm, or “cone”, suspended within a speaker frame, or “basket” around the outer edge with a flexible membrane, or “surround”. This membrane allows the cone to move inward and outward when driven by a varying magnetic field resulting from the application of an audio, or “music”, signal applied to the speaker.
Over the years speakers have been designed with a conventional structure—a cone connected to the outer part to a speaker frame, or basket, through a flexible membrane (surround). To develop a back-pressure wave and to control axial movement of the cone, designer installed a secondary part called a “spider” that also connects the inner part of the cone to the speaker frame. Almost all spider materials used are made of cloth that has been treated and pressed in a heated die to form the shape of the spider that was sought. Conventional speakers require a huge mounting depth that render them useless in shallow spaces where consumers now wish to place speakers. For example, a conventional 10″ diameter speaker, with an excursion of +/−1″ requires a mounting depth of at least 7″. Moreover 12″ diameter conventional speakers requires a mounting depth of at least 7″ to 8″. Hence conventional speakers clearly will not fit in shallow spaces, such as walls where the mounting depth is limited to about 3.5″, or less, unless a smaller diameter conventional speaker is used. Thus, consumer demand has created a need that conventional speakers can not meet and still provide the performance desired by the consumer. Therefore there is a need to develop loudspeakers that have a large piston area with a minimum mounting depth. Low profile speakers designed using the present invention meet that need.
Conventional speakers have many weaknesses that have become much more evident in longer stroke woofers. Since conventional speakers rely upon the glue ring connection of the cone with the voice coil bobbin and spider, that connection is subjected to bending moments that collapse the glue ring during downward (inner stroke movements) and flare outward the glue ring during outward strokes. Additionally, the structure of conventional speakers promotes harmonically related bending of the cone during inward/outward strokes that fatigues the inner portion of the cone and leads into what is known as a neck-cone failure. This typically, partially or completely, breaks the cone into two cones around the neck area. Prior to that type of failure the cone is known to have a cycle per life during which the cone is breaking down and during the slow breakdown of the cone, the conventional promotes increasing distortion that is increasingly unpleasant for the listener. Further conventional speakers have not been designed to maintain the inner suspension (spider) parallel to the outer suspension (surround) as the cone is driven by the voice coil. The spider and surround are each rigidly connected to the inner and outer edges of the cone, respectively, and any misalignment of those connections and/or variations in the material of the spider, surround and cone around the speaker cause the cone to twist in opposite directions as it is driven inward and outward, with the amount of that twisting increasing as the stroke of the voice coil bobbin increases in each direction. This connection configuration can only connection can only compromise such a structure this as the cone bends as it is moves and causes the twisting, or spiraling movement.
Another problem that results in reduced audio performance of conventional speakers is wobble of the voice coil during operation of the speaker. Current speaker design structures suffer from several compromising parts that play a major role in producing a high level of harmonic distortion. As it has been a trend in speaker design to get the most output out of a speaker opening, they resort to increasing the excursion in order to increase the amount air displacement. What previously was a 0.3″ high voice coil are now 1.5″ and as high as 2″ winding heights of the voice coils. These increased height voice coils thus move in excess of 1″ each way, inward and outward. Often speakers can be found where the movement is as much as 1.5″ each way. During extreme excursions, these woofers are pushed by these long voice coils that weigh three times as much as in previous designs. The motor (voice coil) is connected to the cone and the spider in what is known as the inner suspension.
The cone is the stiff component relative to the suspension and surround, extending outward (generally) and connects the inner suspension to an outer larger diameter suspension. The combination of spider, cone, outer surround, and voice coil bobbin are interconnected to oscillate axially. When an audio signal with a frequency F is sent to the voice coil it develops a variable magnetic field that interacts with the fixed magnetic filed produced by the magnet assembly to produce an oscillating force. During these oscillations, the moving parts are subjected to a uniform internal pressure due to the compressed air in the enclosure and tension developed by the spider and surround. The spider and surround each have some manufacturing offset that tend to be apparent during long strokes as the moving elements will start to wobble. The cone typically is made of processed materials (e.g., pressed paper) thus the cone also possesses a non linear stiffness that leads to another offset. The combination of these offsets leads to wobble of the voice coil bobbin.
That wobble can distort the sound produced in varying degrees as the voice coil travels inward and outward in many ways, e.g., distorting the shape of the cone. Wobble can also reduce the useful life of a speaker by repeatedly over stressing the cone and other components that eventually results in failure of the component, e.g., a crack or a tear in the cone, partial separation of the cone and surround, etc. Wobble can even result in total failure of the speaker. This can occur if the voice coil is over driven outward with the lower edge of the voice coil bobbin coming completely out of the magnet assembly with the wobble shifting the lower edge of the voice coil bobbin so that it is no longer aligned with the slot in the magnet assembly. The bottom edge of the voice coil bobbin then hangs up on the top of the magnet assembly as the tension in the spider and surround pull the cone and attached voice coil bobbin downward when the lower end of the voice coil bobbin does not reenter the magnet assembly. Once hung up on the top of the magnet assembly the speaker can no longer move regardless of whatever drive signal is applied to the voice coil since the voice coil is no longer in the magnetic field of the magnet assembly so the drive signal does not interact with the magnetic field, i.e., no signal when applied to the voice coil will be able to move the voice coil bobbin.